Monographs Details: Isopterygium
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants small to medium-sized, in often lustrous, mostly soft, pale- to yellow-green, sometimes brown-tinged, thin to dense mats. Stems creeping, almost simple or irregularly but freely branched, often obscurely complanate-foliate; in cross-section without a hyalodermis, with small thick-walled cells surrounding larger thinner-walled cells, central strand usually absent; pseudoparaphyllia filamentous or rarely absent by reduction; axillary hairs with a single relatively long brown basal cell and 1-3 elongate hyaline distal cells. Stem and branch leaves similar, crowded to lax, usually complanate, erect to erect-spreading to wide-spreading, symmetric or asymmetric, lanceolate to ovate, acute to acuminate, mostly gradually so, concave or flat, nondecurrent or short-decurrent by 1-2 cells; margins entire to serrulate above, subentire to entire below, plane to erect or incurved, often recurved below; costa short and double or absent; cells linear, often flexuose, smooth, thin- to firm-walled, not porose, not becoming shorter in the acumen, becoming shorter, ± rectangular and sometimes porose toward the insertion; alar cells differentiated, few, quadrate to short-rectangular, in extreme angles, not reaching the costa. Asexual propagula rare, clustered on stems, filamentous, uniseriate, simple or branched, the cells roughened. Mostly autoicous. Perichaetial leaves erect, oblong-lanceolate, gradually or abruptly acuminate; margins entire to serrulate, plane or erect or incurved; costa usually none; cells linear, smooth, thin- to thick-walled, becoming shorter and broader toward the insertion; alar cells not differentiated. Setae elongate, smooth, reddish, twisted, straight or curved just below the urn; capsules erect to inclined or pendent, straight to arcuate, cylindric to short-cylindric, constricted below the mouth when dry; exothecial cells quadrate to rectangular, firm-walled, the crosswalls often thinner than the vertical walls, sometimes collenchymatous; annulus not differentiated; operculum conic to short conic-rostrate; peristome double, exostome teeth shouldered, bordered, on the front surface cross-striolate below, coarsely papillose above, trabeculate at back; endostome with a low to high basal membrane, segments narrow to broad, keeled, not or narrowly perforate, shorter than or ca. as long as the teeth, cilia absent or in groups of 1-3, usually papillose. Spores spherical, smooth to papillose. Calyptrae cucullate, naked, smooth.
Discussion:Isopterygium Mitt., J. Linn. Soc., Bot. 12: 21. 1869; Plagiothecium subgen. Isopterygium (Mitt.) Grout, Moss Fl. N. Amer. 3: 162. 1932.
Hypnum sect. Omalia subsect. Taxicaulis Müll. Hal., Syn. Musc. Frond. 2: 277. 1851; Taxicaulis (Müll. Hal.) Müll. Hal., Bot. Jahrb. Syst. 23: 329. 1896.
Discussion. Isopterygium is often a difficult genus to recognize. The plants are usually ± complanate and often pale green, presumably adapted to low light intensities. The leaves are often somewhat asymmetric. The filamentous pseudoparaphyllia are often the best way to be sure of identification. They separate Isopterygium from Taxiphyllum in which the pseudoparaphyllia are foliose. Additionally, in Taxiphyllum the apical leaf cells are often shorter than the median ones, a situation never found in Isopterygium. Isopterygium always has a few differentiated alar cells; Taxiphyllum usually has somewhat more. In Isopterygium most, but not all, species are autoicous, whereas in Taxiphyllum the species are uniformly dioicous.
Although pseudoparaphyllia are sometimes listed as absent in some species of Isopterygium, it is really a difference in branching that is occurring. In those species for which pseudoparaphyllia are reportedly absent, the branching is usually sparse and irregular. In these species the apical meristem seems to only rarely produce branch primordia and branches are produced when the stem apex dies or is broken off, which, in some species, seems to occur rather frequently. In these species, an examination of branch bases reveals that the branches are produced in leaf axils and are often contracted at the point of attachment to the stem. However, in genera that are supposedly void of pseudoparaphyllia, a rare branch primordium or young branch can usually be found with true pseudoparaphyllia. To me, at least, this indicates that evolutionarily these plants once produced typical branch primordia but it became more advantageous to put most of the plant’s energy into singly-directed forward growth, rather than dispersed lateral growth. Based upon this premise, I am more inclined to be swayed by differences in the morphology of pseudoparaphyllia than merely by their presence or absence. However, Iwatsuki (1987) recently described Pseudotaxiphyllum (q.v.), distinguished by the lack of pseudoparaphyllia, as well as other correlated features, such as different propagula, rhizoid position, dioicous sexuality, and the presence of capsular annuli.
Axillary hairs are decidedly difficult to find in Isopterygium. The problem seems to be a result of several combined factors: numerous pseudoparaphyllia that obscure axillary hairs, relatively few hairs per leaf axil (only a single one?), and hairs that seem to wither earlier than in most other pleurocarps.
When Mitten (1869) described Isopterygium, he included eight species in the genus. E. G. Britton (1918) listed I. planissimum Mitt. as the type of the genus. However, she seems only to have chosen the first species listed by Mitten and, indeed, her choice is now considered a synonym of Taxiphyllum taxirameum. Likewise, Ireland’s (1969: 47) choice of I. pulchellum (Hedw.) A. Jaeger as a lectotype of the genus is unusable because the species was not included among those species when the genus was described. Finally, Iwatsuki and Crosby (1979) chose I. tenerum (Sw.) Mitt. as the lectotype. It is fully in accord with the modem understanding of the genus.
Ireland (1991, 1992) has published two accounts of Isopterygium in Latin America. I have defined species somewhat more narrowly, but for an extralimital account of the genus, his publications are well worth consulting.